rapid eye movement

When your head hits the pillow, your eyes still function. "But they can only sense light versus dark," says physician Michael Breus, a clinical psychologist who founded SoundSleepSolutions.com, a sleep-information Web site. This explains why a bright light or the sunrise often wakes a person up.

At what level of the animal hierarchy do we find true sleep?
Rich Sefcik
Buffalo Grove, Ill.